Security systems are presently limited in their ability to detect contraband, weapons, explosives, and other dangerous objects concealed under clothing. Metal detectors and chemical sniffers are commonly used for the detection of large metal objects and some types of explosives; however, a wide range of dangerous objects exist that cannot be detected with these devices. Plastic and ceramic weapons are non-metallic objects that security personnel are required to detect. Currently, existing systems do not detect such objects well, and the alternative of manual searching is slow, inconvenient, and not well tolerated by the general public, especially as a standard procedure in, for example, airports.
Prior art X-ray systems for detecting objects concealed on persons do exist and can be improved from the perspective of radiation dosing, scanning speed and image quality. For example, U.S. Pat. No. 5,181,234 (hereinafter, the “'234 patent”), assigned to the assignee of the present invention, and herein incorporated by reference in its entirety, describes “[a] pencil beam of X-rays is scanned over the surface of the body of a person being examined. X-rays that are scattered or reflected from the subject's body are detected by a detector. The signal produced by this scattered X-ray detector in then used to modulate an image display device to produce an image of the subject and any concealed objects carried by the subject. The detector assembly is constructed in a configuration to automatically and uniformly enhance the image edges of low atomic number (low Z) concealed objects to facilitate their detection. A storage means is provided by which previously acquired images can be compared with the present image for analyzing variances in similarities with the present image, and provides means for creating a generic representation of the body being examined while suppressing anatomical features of the subject to minimize invasion of the subject's privacy.” The system, as described, however, requires that the subject assume at least two poses for a full scan. Even with the at least two poses, certain areas of the subject may not be captured due to hidden areas. In addition, the scanning system of the '234 patent is only capable of detecting backscattered radiation, due to the position of the detector array. This patent is incorporated by reference.
In addition, U.S. Pat. No. 6,094,472 (hereinafter, the “'472 patent”), also assigned to the assignee of the present invention, describes a method for using an X-ray backscatter imaging system for searching a subject for concealed objects, “comprising the steps of: moving the subject within a passageway, the passageway having an entrance and an exit; initiating operation of at least one X-ray source upon entry of the subject into the passageway; producing a pencil beam of X-rays having a low dose directed toward a scanning area at a plurality of scanning positions within the passageway; scanning the pencil beam of X-rays over the scanning area; tracking said pencil beam of X-rays to each of said plurality of scanning positions, wherein the tracking is substantially coordinated with forward progress of the subject through the passageway; using a plurality of detectors, detecting X-rays that are backscattered from said pencil beam as a result of interacting with the subject when positioned at each scanning position of the plurality of scanning positions; and displaying a digitally represented image of the detected backscattered X-rays.” The '472 patent suffers from the same disadvantages as the '234 patent in that it is only capable of detecting backscattered radiation, due to the positioning of the detector array and radiation sources and requires that the subject move through the passageway of the system, thus complicating detection capability due to the motion of the subject. This patent is incorporated by reference.
Further, U.S. Pat. No. 6,393,095 (hereinafter the “'095 patent), issued to Robinson, describes a method for detecting a defect in a sample in which “a pair of stereoscopic images of the sample are produced, one image is subtracted from the other to provide a resulting two dimensional image, and that said resulting two dimensional image is checked against a golden two dimensional image which has been produced in identical fashion from the said golden sample, identity of the two images indicating identity between the said sample and the said golden sample, differences indicating a difference between the said sample and the said golden sample.” The '095 patent, however, uses methods of subtracting resultant images, rather than combining images. This patent is incorporated by reference.
The above-mentioned systems, while effective at people screening, have low overall throughput and require several scanning or image acquisition steps. In addition, they require large amounts of operational space, which is not always available at a given inspection site.
Accordingly, there is need for a relatively compact people screening system that is capable of scanning both the front side and back side of a person without requiring the person to rotate or walk through for additional scans, thereby enabling relatively higher overall system throughput.
Further, conventional people screening systems generally contend with the competing interests of image quality and privacy, usually at the expense of either one of the two. Thus, there is a need for a people screening system that uses a combination of backscatter image signals with shadow image signals to provide higher quality images for better edge enhancement and contraband detection without invading privacy of the person.